1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing electric resistance welded tubes.
2. Description of the Prior Art
A high-frequency resistance welding or a high-frequency induction welding are generally used for making electric resistance welded tubes.
In such weldings, a flow of current is concentrated to a rather narrow band in each of mating edges which are brought in an "angle of convergence" or so-called "Vee-angle". This phenomenon is often called "Skin effect". An inner and an outer edge of a hoop have higher temperatures than a center in thickness of tube has. The difference in temperatures between the inner and outer edges of the tube and the center in thickness is remarkable particularly on a heavy walled tube. Therefore, for heavy wall, even when the center of the mating surface are not heated to a forging temperature, the edges are tend to reach a melting temperature. When the edges are heated over the melting point, an oxide easily appears thereon which is one of causes for welding-defects. To avoid such welding-defects caused by an oxide, forging pressure is increased for urging out oxide from a welding-skin. The increase of the pressure is not desirable, because it causes the increase of metal flow angle and reduces impact-characteristics. The increase of metal flow also deteriorate the performances of flattening test and flaring test. And, in addition, non-welded part is apt to appear near a surface of a weld line since the molten metal is not push out from the mating surfaces due to the insufficient pressure.
Consequently the prior art is inadequate for welding extremely heavy walled tube efficiently and limit the thickness to an extent of 12.7 mm. Submerged welding and TIG welding is often used for a heavier walled tube, but the yield efficiency of which are less than one-third of high-frequency welding. Recently in the production of a tube, electric welded tubes of 16 mm wall thickness are manufactured by lowering a line speed to an extent that the center of the mating surfaces are sufficiently heated through the heatdiffusion. In such art, a welded surface is easily oxidized to cause defects because of a longer allowable time for oxidation. In the production of a high-quality tube, e.g., a thick line pipe and etc., there may be utilized a gas-sealing on a welding-zone to prevent the welding-part from oxidation and producing defects. However in such art, yield efficiency is rather low because it needs to reduce a line-speed to control a temperature of the center in thickness of the plate to suit for butting.